mds: use inode_t::layout for dir layout policy

Remove the default_file_layout struct, which was just a ceph_file_layout,
and store it in the inode_t.  Rip out all the annoying code that put this
on the heap.

To aid in this usage, add a clear_layout() function to inode_t.

Signed-off-by: Sage Weil <sage.weil@dreamhost.com>
Signed-off-by: Greg Farnum <greg@inktank.com>